The field of the invention is aromatic carboxylic acids, and particularly the method of preparing phthalic, isophthalic and terephthalic acids by the liquid phase air oxidation of the corresponding alkyl benzenes dissolved in lower carboxylic acids and in the presence of a bromine activated heavy metal catalyst system.
The state of the prior art may be ascertained by reference to the Kirk-Othmer "Encyclopedia of Chemical Technology", 2nd Edition, Vol. 15, pages 444-487 under the section "Phthalic Acids". Kirk-Othmer points out, at page 451, that one of the four fundamentally different reaction processes for preparing phthalic acids is a liquid phase air oxidation process based upon a solvent system such as acetic acid and with a bromine activated heavy metal catalyst system. This process is based upon the oxidation of mixed xylenes to mixed phthalic, isophthalic and terephthalic acids. Phthalic, isophthalic and terephthalic acids are also called respectively, ortho-, meta- and parabenzene dicarboxylic acids.
The state of the prior art is also shown by U.S. Pat. Nos. 2,245,528 of Loder; 2,276,774 of Henke; 2,415,800 and 2,833,816; West German Printed Specifications (DAS) Nos. 1,004,159; 1,081,445; 1,130,447; 1,168,867; 1,210,790; and 1,235,887; East German Pat. No. 10,918; and the reference Chem. Ing. Techn. 34,51 (1962).
Ever since the disclosure of the U.S. patent of Loder, it has been generally known that toluic and phthalic acids can be obtained by the air oxidation of xylenes dissolved in lower carboxylic acids when certain heavy metal compounds are used as catalysts, preferably, among others, compounds of vanadium, cerium, cobalt and manganese.
The U.S. patent of Henke suggested performing the oxidation also in the presence of barium or lead compounds. Optimum results were first obtained with the addition of barium and lead bromide as suggested by Henke and others if, in accordance with U.S. Pat. No. 2,415,800 and East German Pat. No. 10,918, hydrogen bromide was used to promote continuous oxidation to the final end product.
Ever since these early teachings, the air oxidation process has been repeatedly described and modified. German Printed Specifications Nos. 1,081,445; 1,130,447; as well as 1,210,790, and U.S. Pat. No. 2,833,816, and also, for example, Chem. Ing. Techn. 34,51 (1962), show that catalytic systems which contain manganese are especially effective. Thus, in German Printed Specification No. 1,004,159, Example 3, it is clearly stated that manganese produces a much higher yield of a better product than is obtained with cobalt.
It has been observed that phthalic acids which are formed by the known air oxidation processes, and especially the terephthalic acid, are extremely finely granular. This is especially true of those terephthalic acids which are produced from p-xylene. Thus, German Printed Specification No. 1,235,887 shows in its examples that, depending on the specific method of preparation, there are obtained average terephthalic acid particle sizes of only 0.5 to 2.0.mu..
These fine suspensions settle very slowly and the separation and processing of these suspensions presents great difficulty, especially during large quantity production. Such a finely suspended acid cannot be separated in the standard centrifuges or decanters. A finely grained acid can only be isolated by using standard filtration methods with the use of suction filters or filter presses. According to the German Printed Specification No. 1,235,887, the separation is performed in a rotary filter.
There are also serious difficulties with the filtration methods because the very small particle sizes form a filter cake which is practically impermeable. The grains are deposited in the pores of the filter medium and this greatly retards the filtration and necessitates the use of unusually large filtration surfaces. It is also to be considered that the diminution of filter output by the fine particles is caused not only by the obstruction of the pores in the filter medium, but also in a large measure by the absorption of the mother liquor by the surface forces of the filter material. These surface forces become very prominent because of the rapid increase of surface area which results from diminishing particle size. These surface forces impose a limit on the dewatering and washing of the filter cake. In the process disclosed in German Printed Specification No. 1,235,887, the separated phthalic acid has a residual moisture content greater than 50%, which frequently renders the filtration product thixotropic and makes the washing and drying very expensive. The above-identified problem has been pointed out in German Printed Specification No. 1,047,192 and in this disclosure an effort has been made to solve the problem for the production of phthalic acid by oxidation with nitric acid.
For those phthalic acids which are obtained by the methods of Loder, Henke and others, the problem still remains unsolved.